This invention relates to a method for the production of reinforced composites from compositions which are photopolymerisable and thermosettable and from fibrous reinforcing materials, and to the composites obtained by this method.
Composite structures are commonly made by impregnating fibrous materials, such as paper, glass, and carbon fibres, with a solution of a solid thermosettable resin and a heat-activated curing agent for the resin, causing the resin to solidify by evaporation of the solvent, and, when desired, curing the resin composition by the action of heat. Composite structures may also be prepared from films of a thermosettable resin composition by laying a film of the resin composition on a fibrous reinforcement and applying heat and pressure so that the resin composition flows about the fibres but remains curable, and then heating further when desired so that the resin composition is cured by the heat-activated curing agent.
Both these methods suffer from certain drawbacks. If a solvent is used, it is not always possible to eliminate all traces of it before the final curing takes place, and in consequence the final composite may contain voids caused by evaporation of such residual solvent. Further, use of solvents may cause difficulties due to their toxicity or inflammability or to pollution. When a film adhesive is used, it must first be cast from a liquid thermosettable resin and this then advanced to the solid state, and such a process adds considerably to the cost of the composite. Both methods also require a considerable expenditure of heat energy, either to evaporate the solvents or to advance the resin.
Japanese published patent application No. 113498/72 disclosed a process in which glass fibres were impregnated with glycidyl methacrylate or glycidyl acrylate in the presence of a ring-opening catalyst, optionally also with other vinyl monomers or prepolymers, the fibres were subjected to irradiation at low temperature with an ionising radiation or light, and then the composite was heated to cure it. In the examples, glasscloth impregnated with glycidyl methacrylate is subjected to gamma radiation from .sup.60 Co at from 4 .times. 10.sup.5 R/hour to 1 .times. 10.sup.6 R/hour for 11/2 to 3 hours. Use of light to induce polymerisation is not discussed, other than to say that visible or ultra-violet light may be employed. The need to apply gamma or similar ionising radiation for such prolonged periods clearly limits the industrial usefulness of the process.
We have now found a method by which reinforcing materials may be impregnated with a liquid, solvent-free composition and this composition rapidly converted into a solid, but still heat-curable, state without the inconveniences just mentioned of the prior art methods. In this novel method, a liquid composition, containing an epoxide resin and a photopolymerisable compound, it photopolymerised by exposure to actinic radiation, optionally in the presence of a catalyst for the photopolymerisation, but without thermally crosslinking it; exposure times as short as one second have been used successfully. The resultant prepreg is, when desired, fully cured by heating to form the composite.